Dithering using PHD2 and BYEOS

[Diego Gomez]

Hello All,

I hope you guys can help me with the following:

I've been reading about dithering setup when using PHD2 and BYEOS and I think I am pretty much clear in how it's work. The thing is that I have two different optical setup and therefore the way how they are dithered should be different.

In this case, I am thinking in having different dither scales in PDH2 (one for each setup) while keeping the BYEOS dither aggressiveness fixed.In this case I don't need to be worry about changing parameters in both systems but in one of them.

In my specific case:

SETUP 1:

- Main scope: 9.25EdgeHD with 0.7 focal reducer ----> FL: 1645 mm

- Main camera: EOS 550D: 4.3um pixel size

- Arc-sec/pixel = 0.54

- Guide scope: 60mm f3.6  ----> 216 mm

- Guide camera: Orion SSAG: 5.2um pixel size

- Arc-sec/pixel = 4.99

Then, imaging scale = 4.99 / 0.54 = 9.2

If I want to dither 13 pix on the imager  ---> 13 / 9.2 = 1.41 pix

PHD2 dither scale should be 2

BYEOS dither aggressiveness should be 1

SETUP 2:

- Main scope: SW Esprit 100ED ----> FL: 550 mm

- Main camera: EOS 550D: 4.3um pixel size

- Arc-sec/pixel = 1.61

- Guide scope: 50mm f4.1  ----> 207 mm

- Guide camera: Orion SSAG: 5.2um pixel size

- Arc-sec/pixel = 5.18

Then, imaging scale = 5.18 / 1.61 = 3.2

If I want to dither 13 pix on the imager  ---> 13 / 3.2 = 4.1 pix

PHD2 dither scale should be 4

BYEOS dither aggressiveness should be 1

Do the calculations above make sense?

Many thanks & clear skies,

Diego Gomez

Calgary, AB

[Andy Galasso]

Hi Diego,

Those calculations look right to me, but not sure how you got the dither scale values of 2 and 4 (rounding?) The dither scale can be a fractional number and does not need to be a whole number, so you could set it to 1.4 and 4.1 if you wanted. Then a 1 pixel dither from BYEOS would get scaled to 1.4 px or 4.1 px in phd2.

Andy

[Diego Gomez]

HI Andy,

Thanks a lot for looking after this! I wasn't sure whether or not I could use decimals in the PHD2 dither scale. I'll use the exact values and I will give it a try.

Cheers,

Diego

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[John Sim]

I don't mean to hijack this thread but it reminded me of a question/suggestion I had about dithering. I too have been using BYEOS with PHD2 to dither between subs. My understanding is that the combo of the BYEOS' aggressiveness parameter and PHD2's dither scale define the Maximum dither amount and the actual dither is a random value in that range. E.g. If BYEOS aggressiveness was 5 and PHD2 dither scale was 4 then the magnitude of the dither would be a random value in the range 0 to 20, with an average dither of 10 pixels; the direction of dither is random too. For DSLR and cooled CMOS cameras the Fixed Pattern Noise can only reliably be avoided by dithering and I've seen recommendations for the required amount to dither ranging from 5 pixels to 12.5 pixels.

On my imaging runs I found that the mount took a long time to recover the lock position after a large dither - often exceeding the timeout - and for the really teeny dithers I often wondered if they were achieving anything. So my question is could PHD2 have a min and max dither scale? E.g. if the min scale was 1, max scale 3 and BYEOS aggressiveness was 5 then the dither range would be a random number between 5 and 15, with an average of 10. I.e. there's now a minimum guaranteed dither between frames and the maximum dither can be reduced, hopefully leading to a faster dither recovery and a less stressful imaging session.

cheers

John

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[bw_msgboard]

Hi John.  Have you tried using spiral dither?  We added that primarily for users who are dealing with fixed pattern noise.  The spiral dithering moves in a predictable way, spiraling outward (clockwise) in increasing rings.

 

Bruce

 

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[John Sim]

Hi Bruce

I've been using an Astrotrac mount which can only dither in RA - I don't think spiral dithering is applicable in that case? I do have a new EQ6 mount that I will use (waiting for the rainy season to end and the skies to clear) and will be able to use spiral dithering with that. The Astrotrac will become my second rig. I have not yet mastered guiding with the Astrotrac and still haven't found the settings for consistent performance but will persevere and if I succeed then my issue with large dithers (the excessive recovery time) may disappear.

cheers

John

[bw_msgboard]

Hi John.  Although it’s counter-intuitive, it should work in RA-only mode.  You’ll just get increasing moves in the +/- RA direction – like {0,1,-1,-2,2,3,-3,-4,4,5,...} multiplied by requested_amount * scale_factor.

 

So I guess you’d need to be careful that this didn’t become too large.  That said, I think you’ll need to use a 2-axis mount to really get anywhere.

 

Good luck,